How Did That Happen?

I seem to have changed some setting on my MacBook such that when I try to add anything to the Applications folder at the all-user level, I have to authenticate my account (even though the user account has admin privileges). This could have resulted from one of the system updates over the past few weeks, I’m not sure; does anyone else have an idea of what happened?

[Later: I had checked permissions on the Applications folder before, but I think I had checked only the Applications folder in my user directory, not the Applications folder at the system level. When I went back to check it, to avoid being made to feel stupid when someone suggested that, it was locked — so I unlocked it, which I think may solve the problem.]

[Even later: And the winner is — Kevin! He writes,

AKMA,

I had this problem after the 10.4.10 update.

I fixed it by booting off the install DVD and running repair permissions on the boot volume.